MLB starting pitchers play every 5 games. Do they get paid only for each start they make or do they earn for each game in between starts as well?
All Major league baseball players are paid at a daily rate equivalent to their annual salary; that daily rate is calculated based on the Championship Season, which is defined as the time from Opening Day to the final day of the "regular" season (so, usually around Sept. 30). It is between 178 and 183 days in length. So, if the player is paid $1MM, and this year has 183 days in it, they are paid $5,464 per day. This distinction (between "days" and "games") is important both for players traded (who might play more than 162 games) and suspended (you lose your salary for the duration of the suspension, based on calendar days).
The rate is regardless of games played; you get paid the same for being on the bench as on the field. In fact, if you have a Major League contract (and not a split contract), you get paid the same even in the minors. (Most players in their first six years are on effectively split contracts, as are many players who are either not good enough to make the majors as a regular ("AAAA" players) or who are no longer good enough may also agree to split contracts that pay differently based on whether they are on the Major League 25 man roster or not.)
That payment is then made in semi-monthly installments during the championship season (ie, as opposed to many teachers and some other similar professions, baseball players receive all of their salary during the six or so months they play - they don't have to wait six months for the rest of it.)
This is spelled out in the Uniform Player Contract.
In accordance with Paragraph 2 of the Uniform Player's Contract, for each day of service while in the Major Leagues, Club shall pay the Player a salary (...), at a rate and for the season(s) indicated below in the following manner:
(The linked contract is a split contract, so that player does receive a different salary at different levels.)
Because of the nature of the collective bargaining agreement, starting pitchers are rarely treated any differently than any other player. There are some occasional differences between pitchers and position players (like the length of rehab stints). However, paychecks is not one of those places.
They earn their checks based on the amount of time they are on the roster. MLB players get paid on the 1st and 15th, unlike in other sports that play weekly where you get paid weekly (like the NFL). Most MLB contracts are completely garunteed though so you get the full amount of your contract regardless of roster status (rookie contracts are different though).
This means that in general, a players contract value will be divided up by the number of pay periods during the season (12 or so), and they would get paid that amount each check.